In printing technology it is known to print, on the one hand, web-shaped substrates and, on the other hand, individual, sheet-shaped substrates. When printing a web-shaped substrate, the substrate is rolled off a roll and moved past one or more printing units of a printing machine, where a printing medium such as, for example, ink is applied to the web-shaped substrate.
In order to be able to ensure a uniform printed image, a substrate should always be moved through the printing machine in a known position, if possible. In particular in duplex-printing, when the substrate is turned over and repositioned, it should be ensured that the position of the substrate is known when printing the recto side, as well as when printing the verso side.
In order to be able to position the substrate, it is initially necessary to define the position of the substrate within the printing machine.
This may be accomplished, for example, by optical systems that parallelize the light of a light source in a collimator and, as a result of this, create a shadow image of a substrate edge on a sensor. FIG. 10 shows a schematic side view of such a known device 130. The device 130 comprises a light source 135, a collimator 140, as well as a CCD sensor line 145. The sensor line 145 is arranged in such a manner that plane-parallel light from the collimator 140 can impinge on the sensor line 145. A substrate 150 located between the collimator 140 and the sensor line 145 can partially cover the CCD sensor line 145. The degree of coverage can be used to determine the position of an edge of the substrate 150.
The disadvantage of such an arrangement is the limited length of the sensor units. A large detection range needs to be available in order to be able to detect substrates of greatly varying formats. However, this represents a problem from the viewpoint of the constructional size of the sensors because, as the length of the sensors increases, the constructional size of the collimator also increases significantly and, consequently, the sensor unit as a whole does not only become longer but also significantly more bulky.